1. Field of the Invention
The present invention relates to chalcopyrite compound semiconductors (semiconducting compounds) usable for high-efficiency thin-film solar cells, nonlinear optical elements, or the like.
2. Description of the Prior Art
CuInSe.sub.2 belongs to the I-III-VI.sub.2 family of chalcopyrite semiconducting compounds. This compound is used as an absorber layer in thin-film solar cells which have very high efficiencies and long-term stable characteristics. Compound semiconductors having a chalcopyrite structure in which atoms are replaced with homologous ones, represented by: EQU (I.sub.1-x -I'.sub.x)-(III.sub.1-y -III'.sub.y)-(VI.sub.1-z -VI'.sub.z).sub.2,
wherein I and I', III and III', and VI and VI' are homologous elements belonging to the same groups, respectively, are known. It is known that when some atoms of the I, III, and VI groups constituting the base of a chalcopyrite crystal structure are replaced with homologous ones, electrical, optical, and other various properties of the crystal change.
However, chalcopyrite compound semiconductors in which atoms of a VI-group element are replaced with those of an element of a different group, for example, the V or VII group, represented by I-III-VI.sub.2-x V.sub.x or I-III-VI.sub.2-x VII.sub.x have not been reported.
Generally, when a p-n junction is formed by two semiconductor thin films of chalcopyrite compound represented by: EQU (I.sub.1-x -I'.sub.x)-(III.sub.1-y -III'.sub.y)-(VI.sub.1-z -VI'.sub.z).sub.2,
in which some atoms are replaced homologous ones, or that of a non-stoichiometric chalcopyrite compound represented by: EQU (I.sub.x -III.sub.y)-VI.sub.2,
wherein x+y=2, many defects are produced at the junction because of the difference in the lattice constant, the crystal orientation of grains with respect to the surface of the substrate, and the surface morphology at the junction. As a result, high density recombination centers are produced near the p-n junction, causing deterioration of the properties of resultant semiconductor devices such as solar cells and optical sensors.
One purpose of the present invention is to provide semiconductor devices with improved properties in which the lattice constant, the crystal orientation of grains with regard to the substrate surface, and the surface morphology are matched at the p-n junction in a I-III-VI.sub.2 chalcopyrite compound semiconductor thin films by using a semiconductor thin film of chalcopyrite compounds represented by I-III-VI.sub.2-x -V.sub.x or I-III-VI.sub.2-x -V.sub.IIx. For simplification, hereinafter, the I-III-VI.sub.2 chalcopyrite compounds also include those in which some atoms are replaced with homologous ones and those having a non-stoichiometric composition. Also, the I-III-VI.sub.2-x -V.sub.x or I-III-VI.sub.2-x -VII.sub.x chalcopyrite compounds also include those in which some atoms are replaced with homologous ones and those having a non-stoichiometric composition.
The chalcopyrite compound semiconductor thin films represented by I-III-VI.sub.2 are not good in adhesion to a substrate such as glass or a metal film on the glass. Thus arise a big problem arises when semiconductor devices are actually fabricated.
When semiconductor thin films of conventional chalcopyrite compounds such as CuInSe.sub.2 are used for optoelectro transducers such as solar cells, a heterojunction of CdS/CuInSe.sub.2, for example, is formed as shown in FIG. 1. FIG. 1 shows a structure of a conventional solar cell which comprises a glass substrate 18, and a lower electrode 24 made of Mo, a p-type CuInSe.sub.2 layer 25 having a thickness of 2 .mu.m, an n-type CdS layer 31 having a thickness of 1 .mu.m, and a ZnO layer 22 having a thickness of 2-3 .mu.m formed in this order on the substrate 18. An upper metal electrode 30 is then formed on the ZnO layer 22. This structure causes failure in the lattice matching at the junction and the resultant production of high density recombination centers, which deteriorate the properties of the semiconductor devices.
To solve the aforementioned problem and to improve the properties of semiconductor devices, a yet another purpose of the present invention, which is realized by forming a homojunction in a chalcopyrite compound semiconductor thin film by doping the thin film with molecular beams or ion beams during the formation of the thin film, or by doping the thin film with accelerated ions after the formation of the structure of an optoelectro transducer or the like. Moreover, solar cells, for example, the chalcopyrite compound semiconductor thin films having the homojunction according to the present invention do not contain toxic Cd. Thus, they can contribute to energy saving and environmental protection.